Download SharePoint Designer 2013 from Official Microsoft Download Center.
SharePoint Designer 2013 is the tool of choice for the rapid development of SharePoint applications. Please make sure your installation of SharePoint Designer is up to date by using Microsoft Update, or installing Service Pack 1 see Details for more info. Note: There are multiple files available for this download.

Microsoft SharePoint Designer Wikipedia.
7 On April 21, 2010, SharePoint Designer 2010 was released and made available for download. 8 On October 24, 2011, Microsoft released SharePoint Designer 2007 Service Pack 3. SharePoint Designer 2010, the successor to SharePoint Designer 2007, was released to web on April 21, 2010 in two flavors for IA-32 and x64 CPUs. 10 11 Unlike its predecessor, however, it does not operate in absence of Microsoft SharePoint Server or Microsoft SharePoint Foundation and therefore cannot be used as a generic HTML editor. On October 30, 2012, Microsoft released SharePoint Designer 2013.